The use of thrust augmentors, or afterburners, in conjunction with gas turbine engines for aircraft propulsion purposes is a well known method for achieving elevated levels of thrust from a given size engine. A typical thrust augmentor includes a cylindrical extension disposed immediately downstream of the gas turbine engine exhaust and receiving some or all of the discharged exhaust gases therefrom.
A fuel distribution assembly is located at the upstream end of the cylindrical extension for dispersing a varying amount of fuel into the exhaust gases whereupon a combustion reaction initiated and completed within the downstream portion of the cylindrical extension further raises the temperature and energy content of the exhaust stream for providing the desired additional thrust.
Typical additional components are a flame holder assembly located downstream of the fuel distribution means for maintaining a stable flame in a desired radial plane, an adjustable throat exhaust nozzle disposed downstream of the cylindrical extension, and an igniter means for initiating the combustion reaction in the augmentor. As will be appreciated by those skilled in the art, certain limitations on the operation and design of the above-described thrust augmentor are necessary in order to achieve safe, effective, and stable operation of the engine-augmentor combination.
One such constraint is the requirement that such augmentors provide a varying amount of additional thrust in response to pilot demand. This feature allows operation of the aircraft in flight regimes not normally achievable with a non-thrust augmented engine configuration. For these and other reasons it is desirable to provide an augmentor arrangement having a fuel turndown ratio of up to 10:1 as compared to maximum augmentor fuel flow. Such a wide range of variation in fuel flow rate has led to the use of various fuel distribution structures, including primary and secondary flow fuel nozzles, varying area fuel nozzles, etc.
A second limitation on augmentor operation involves the minimum and maximum fuel-to-air ratios which must be locally present in the augmentor in order to achieve stable and efficient combustion. As will be apparent to those skilled in the combustion art, introducing an insufficient local concentration of fuel will give a fuel-air mixture which is reluctant to light off and may fluctuate, blow out, or otherwise be unstable even if ignition is achieved. Too high a fuel-air ratio in a local area conversely results in incomplete combustion, reduced reaction temperature, and possible unstable operation.
A third constraint in augmentor operation is the impact of lighting off an augmentor on the upstream gas turbine engine. Augmentors achieve increased thrust by accelerating the engine exhaust gases immediately downstream of the engine. The initiation of combustion in the cylindrical augmentor produces a temporary pressure increase which can cause surging in the upstream fan in an augmented ducted bypass turbofan type engine. It is thus important to incrementally stage and limit the initiation and addition of fuel to the augmentor to avoid producing an undesirable pressure spike at the gas turbine engine exhaust.
Prior art augmentor fuel distribution schemes have addressed these needs by providing a plurality of annular, concentrically staged distribution zones or sub-areas wherein initial augmentor light off is accomplished by first providing fuel to only one of the concentric sub-areas of the total augmentor flow area, and by sequentially distributing additional fuel in adjoining concentric annular sub-areas as the rate of fuel flow is increased to satisfy demanded augmentor thrust. Full augmentor thrust and fuel flow is achieved by finally distributing fuel to the last annular sub-area disposed immediately inwardly adjacent to the interior surface of the wall of the cylindrical extension.
Examples in the prior art of such concentrically staged augmentation fuel distribution methods are disclosed in U.S. Pat. No. 3,698,186 issued Oct. 17, 1972 to Beane et al and U.S. Pat. No. 3,719,042 issued Mar. 6, 1973 to Chamberlain. One of the benefits of using concentrically staged annular sub-areas is the avoidance of initiating a combustion reaction adjacent the augmentor interior surface until only such time as maximum augmentor thrust is required. Such an operating method reduces the exposure of the inner surface to the high temperature augmentor combustion products, lengthening service life and avoiding costly and time consuming replacements.
Chamberlain utilizes a plurality of concentric annular fuel spraybars to deliver fuel to concentrically staged sub-areas. Beane et al shows the use of a plurality of radially extending spraybars which are individually easily removable for service or replacement. As will be appreciated by a review of the Beane et al patent, the number of fuel conduits in the radially innermost extending group of fuel spraybars is equal to the number of concentric fuel distribution sub-areas employed by the fuel distribution scheme. Thus, a four stage concentric fuel distribution scheme requires a radial spraybar having at least four internal fuel conduits, and, in the case of a primary-secondary fuel distribution scheme, eight conduits are required. This large number of conduits necessary in each full length radial spraybar adds to the complexity of the spraybar system and increases the likelihood of failure or other malfunction.
The spraybar system according to Beane also utilizes three differeing length spraybars to achieve the staged concentric fuel distribution, decreasing parts standardization for a given engine. What is needed is a method of distributing fuel over the gas flow area of a thrust augmentor which operates within the above-identified constraints but which does not require the use of complicated and hard-to-service fuel distributing apparatus.